This invention is directed to a new two-layer membrane switch. This switch may be used to define keyswitches for use in keyboards, particularly keyboards where a low keyswitch profile is desirable.
In particular, this invention utilizes two conductive membranes of a type commonly used in membrane keyboards and other applications, in combination with a conductive elastomeric contact, wherein the elastomeric contact is used to contact predefined areas on each of the two conductive membrane layers, thereby providing a circuit path between these layers and defining a switch. In this switch configuration, the conductive portions on both membrane layers are aligned in the same direction. The result is that these membranes can be stacked directly on top of one another without the need for a separate dielectric spacer.
In related art, contact-type membrane switches have been utilized which have two conductive layers as illustrated in FIG. 1. In FIG. 1, there are two conductive membranes, conductive membrane 2 and conductive membrane 4. Each membrane has a conductor attached to one surface, conductor 6 and conductor 8 respectively. Conductive membrane 2 and conductive membrane 4 are separated by a dielectric spacer 10. Switch contact is made by applying a downward force on one of the conductive membranes; in this illustration, conductive membrane 2. This force can be applied in a number of ways. When such switches are used in keyboard applications the force is typically applied via a keystem 12, passing through keystem guide 14. Keystem 12 is typically attached to a keycap, and is actuated by a user depressing the keycap associated with keystem 12 with his finger. Upon actuation, conductor 6 and conductor 8 contact one another, thereby completing an electrical circuit between them. If conductor 6 and conductor 8 are connected to sense and drive circuitry of a type known in the art, the result is a sensed switch output or closure.
The switch structure illustrated in FIG. 1 has been shown to provide reliable switch contacts in keyboard applications, however, this structure also has a number of associated disadvantages.
A first disadvantage has to do with the cost of dielectric spacer 10. Dielectric spacer 10 has both a material cost for the dielectric material, and a cost to assemble the dielectric spacer into the switch configuration, such as in a keyboard assembly.
A second disadvantage associated with the switch structure illustrated in FIG. 1 has to do with limitations imposed on the switch structure by keystem guide 14. In related art keyboards, a plurality of keystem guides 14 are provided on a housing sheet 16. Housing sheet 16 and keystem guides 14 require a certain thickness for implementation, particularly the thickness of housing sheet 16. However, in certain applications, such as keyboards having a low keyswitch profile, it is desirable to minimize the amount of travel necessary for keyswitch actuation. In such applications, it is desirable to eliminate travel constraints, such as those imposed by the thickness of housing sheet 16, while retaining the function of keystem guides 14. One way to accomplish this is to incorporate keystem guides into the base. However, such a keystem guide configuration is not readily adaptable to a switch configuration of the type illustrated in FIG. 1, because of the necessity of keystem guide 14 protruding through membrane 2 and membrane 4.
A third disadvantage associated with the switch structure illustrated in FIG. 1 is that the structure creates a closed space 20 which can cause instability in membranes 2 and 4 in the regions adjacent to closed space 20 when the switch is exposed to varying temperature and humidity conditions. Such instability can affect the performance of switches of this type.